Negative-electrode active material for rechargeable lithium battery

ABSTRACT

The present invention relates to a method for preparing a negative-electrode active material for rechargeable lithium battery, wherein the negative-electrode active material comprises a core comprising material capable of doping and dedoping lithium; and, a carbon layer formed on the surface of the core, wherein the carbon layer has a three dimensional porous structure comprising nanopores regularly ordered on the carbon layer with a pore wall of specific thickness placed therebetween. In some embodiments, the method comprises modifying a material capable of doping and dedoping lithium with an organic functional group, mixing the modified material with an inorganic oxide, heating the mixture, and removing the inorganic oxide to form the negative-electrode active material.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a divisional of U.S. patent application Ser.No. 13/239,912, filed Sep. 22, 2013, now issued as U.S. Pat. No.8,669,008, which is a continuation-in-part of International ApplicationNo. PCT/KR2009/002622 filed on May 18, 2009, published in Korean, whichclaims priority to Korean Patent Application No. 10-2009-0031974 filedon Apr. 13, 2009, and Korean Patent Application No. 10-2009-0042527filed on May 15, 2009, the entire contents of which are incorporatedherein by reference.

BACKGROUND OF THE INVENTION

The present invention relates to negative-electrode active material forrechargeable lithium battery, method of preparing the same, andrechargeable lithium battery comprising the same.

A battery generates electrical power using materials capable ofelectrochemical reaction for positive electrode and negative electrode.A representative example of the battery is rechargeable lithium batterywhich produces electrical energy by change in chemical potential atintercalation/deintercalation of lithium ion in positive electrode andnegative electrode.

The rechargeable lithium battery is prepared using materials capable ofreversible intercalation/deintercalation of lithium ion as positive- andnegative-electrode active materials, by filling organic electrolyte orpolymer electrolyte between the positive electrode and negativeelectrode.

As positive-electrode active material for rechargeable lithium battery,lithium complex metal compound is used, and for examples, complex metaloxides such as LiCoO₂, LiMn₂O₄, LiNiO₂, LiNi_(1-x)Co_(x)O₂ (0<x<1),LiMnO₂, etc. are studied.

As negative-electrode active material for rechargeable lithium battery,carbon materials of various forms including artificial graphite, naturalgraphite and hard carbon capable of intercalation/deintercalation oflithium have been used. Especially, graphite has low discharge voltageof −0.2V vs lithium, and thus, battery using it as negative-electrodeactive material shows high discharge voltage of 3.6V. Thus, the graphiteactive material provides advantage in terms of energy density of lithiumbattery, and secures long life of rechargeable lithium battery due toexcellent reversibility, and thus is most widely used. However, graphiteactive material has problems in that when preparing an electrode plate,capacity is low in terms of energy density per unit volume of theelectrode plate due to low density of graphite (theoretical density 2.2g/cc), and there is riskiness of ignition or explosion due tomis-operation and overcharge of battery because side reaction withorganic electrolyte easily occurs at high discharge voltage.

Thus, inorganic active materials such as Si have been studied. The Siinorganic negative-electrode active material is known to form Li_(4.4)Sito show high lithium capacity of about 4200 mAh/g. However, it causeslarge volume change of 300% or more at intercalation/deintercalation oflithium, namely at charge/discharge. Thereby, pulverization ofnegative-electrode active material occurs, and the pulverized particlesare condensed to cause electrical deintercalation of negative-electrodeactive material from current collector. The electrical deintercalationmay largely decrease capacity retention ratio and cycle life property ofbattery. Therefore, in order to inhibit volume change of inorganicnegative-electrode active material, studies for preparing carbon/Sinanoparticle composite to use it as negative-electrode active materialhave been progressed. In the carbon/Si nanoparticle composite, carbonfunctions as electrical conductor, thus improving capacity retentionratio of battery to some extent. However, in order to obtain relativelyexcellent capacity retention ratio, carbon content should exceed 50 wt %in the composite, which may lower capacity itself, and even if excessiveamount of carbon is included, capacity decreases to less than 1500 mAh/gafter 50 cycles.

BRIEF SUMMARY OF THE INVENTION

The present invention provides negative-electrode active material forrechargeable lithium battery having excellent cycle life property.

Further, the present invention provides a method for preparing thenegative-electrode active material for rechargeable lithium battery.

Furthermore, the present invention provides rechargeable lithium batterycomprising the negative-electrode active material for rechargeablelithium battery

According to one embodiment, the present invention providesnegative-electrode active material for rechargeable lithium batterycomprising: a core comprising material capable of doping and dedopinglithium; and, a carbon layer formed on the surface of the core, whereinthe carbon layer has a three dimensional porous structure comprisingnanopores having average diameter of about 100 to 300 nm, regularlyordered on the carbon layer with a pore wall having thickness of about40 to 150 nm placed therebetween.

The negative-electrode active material for rechargeable lithium batterymay have peak at about 100 eV and/or about 104 eV, and may not have anysubstantial peak at about 105 eV and about 110 eV in the X-rayphotoelectron spectroscopy graph (XPS graph).

In the negative-electrode active material, after conductingcharge/discharge, the average diameter of the nanopores may be about 30to 150 nm, and the thickness of the pore wall between the nanopores maybe about 40 to 120 nm.

The material capable of doping and dedoping lithium may comprise one ormore kinds of Group 14 or 15 element-containing material, selected fromthe group consisting of Si, SiO_(x)(0<x<2), Si—Y₁ alloy, Sn, SnO₂,Sn—Y₂, Sb and Ge (wherein, Y₁ and Y₂ are one or more kinds of atomsselected from the group consisting of alkali metals, alkaline earthmetals, Group 13 atoms, Group 14 atoms, transition metals and rare earthatoms, provided that Y₁ is not Si, and Y₂ is not Sn). And, Y₁ and Y₂ maybe one or more kinds of atoms capable of binding with Si or Sn, selectedfrom the group consisting of Mg, Ca, Sr, Ba, Ra, Sc, Y, Ti, Zr, Hf, Rf,V, Nb, Ta, Db, Cr, Mo, W, Sg, Tc, Re, Bh, Fe, Pb, Ru, Os, Hs, Rh, Ir,Pd, Pt, Cu, Ag, Au, Zn, Cd, B, Al, Ga, Si, Sn, In, Ti, Ge, P, As, Sb,Bi, S, Se, Te, and Po.

The material capable of doping and dedpoing lithium in the core mayexist as multiple particles, and carbon materials may be furthercomprised between the multiple particles.

The core may further comprise an oxide of the material capable of dopingand dedpoing lithium, as well as the material capable of doping anddedpoing lithium.

The material capable of doping and dedpoing lithium may have acrystalline structure, and crystalline grain in the crystallinestructure may have an average diameter of about 20 to 100 nm.

The material capable of doping and dedpoing lithium may have a structurecomprising crystalline grains dispersed in an amorphous matrix, afterconducting charge/discharge. And, the dispersed crystalline grain mayhave an average diameter of about 2 to 5 nm.

The carbon layer may have a thickness of about 1 to 30 nm, and it maycomprise disordered carbon. And, it may have a Raman integratedintensity ratio D/G(I(1360)/I(1580)) of about 0.1 to 2.

The negative-electrode active material may comprise about 5 to 40 wt %of carbon, based on the total amount of the negative-electrode activematerial for rechargeable lithium battery.

The negative-electrode active material may have specific surface area ofabout 50 to 200 m²/g.

According to another embodiment, the present invention provides a methodfor preparing negative-electrode active material for rechargeablelithium battery comprising the steps of: modifying material capable ofdoping and dedoping lithium with organic functional groups; mixing thematerial capable of doping and dedoping lithium modified with theorganic functional group with inorganic oxide; heating the mixture; and,removing the inorganic oxide.

In the method, a composite formed after the heating step may comprise acore comprising the material capable of doping and dedoping lithium; anda carbon layer formed on the surface of the core, wherein the inorganicoxide particle is stuck in the carbon layer.

The organic functional group may be an organic group represented byC_(n)H_(m) (wherein, n and m are integer of 1 or more).

The organic functional group may be selected from the group consistingof aliphatic organic group having carbon number of from 1 to 30,cycloaliphatic organic group having carbon number of from 3 to 30, andaromatic organic group having carbon number of from 6 to 30.

The inorganic oxide may comprise one or more kinds of inactive inorganicoxide, selected from the group consisting of silica, alumina, titania,ceria and zirconia.

The inorganic oxide may be added in an amount of about 10 to 80 parts byweight, based on 100 parts by weight of the material capable of dopingand dedoping lithium modified with the organic functional group.

The material capable of doping and dedoping lithium modified with theorganic functional group may be in the form of viscous gel.

The step of heating can be conducted under vacuum or inert atmosphere.

The step of heating can be conducted at about 700 to 1200° C., and thestep of removing the inorganic oxide can be conducted using basic oracidic material.

According to another embodiment, the present invention providesrechargeable lithium battery comprising a positive electrode comprisingpositive-electrode active material; a negative electrode comprising theabove described negative-electrode active material; and, an electrolyte.

The rechargeable lithium battery may show about 94% or more of coulombicefficiency after conducting 30 cycles or more of charge and discharge.

Other details of embodiments of the present invention are describedherein below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is SEM photograph of negative-electrode active material forrechargeable lithium battery of Reference Example 2.

FIG. 2 is XPS graph of negative-electrode active material forrechargeable lithium battery of Example 1.

FIG. 3a is SEM photograph of composite comprising silica nanoparticlesthat are stuck in a carbon layer formed on the surface of a corecomprising silicon, immediately after heating in the process of Example1.

FIG. 3b is 20-fold magnified view of FIG. 3 a.

FIG. 3c is 100-fold magnified view of FIG. 3 a.

FIG. 4a is SEM photograph of negative-electrode active material forrechargeable lithium battery of Example 1.

FIG. 4b is 20-fold magnified view of FIG. 4 a.

FIG. 4c is SEM photograph of the cross section of negative-electrodeactive material for rechargeable lithium battery of Example 1.

FIG. 5a is SEM photograph of negative-electrode active material forrechargeable lithium battery of Example 1, after conducting 100 cyclesof charge/discharge.

FIG. 5b is cross-sectional view of FIG. 5 a.

FIG. 6a is TEM photograph of the cross section of negative-electrodeactive material for rechargeable lithium battery of Example 1.

FIG. 6b is SADP photograph of negative-electrode active material forrechargeable lithium battery of Example 1.

FIG. 7a is TEM photograph of negative-electrode active material forrechargeable lithium battery of Example 1, after conducting 100 cyclesof charge/discharge.

FIG. 7B is SADP photograph of negative-electrode active material forrechargeable lithium battery of Example 1, after conducting 100 cyclesof charge/discharge.

FIG. 8 shows Raman spectrum of negative-electrode active material forrechargeable lithium battery of Example 1.

FIG. 9 is X-ray diffraction graph of negative-electrode active materialfor rechargeable lithium battery of Example 1.

FIG. 10 is X-ray diffraction graph of negative-electrode active materialfor rechargeable lithium battery of Example 1, after conducting 100cycles of charge/discharge.

FIG. 11 is graph showing the result of isothermal adsorption experimentof negative-electrode active material for rechargeable lithium batteryof Example 1.

FIGS. 12a-12i are graphs respectively showing cycle number vs chargecapacity and Coulombic efficiency of coin-type half cells of Examples6-10, Reference Examples 3 and 4, and Comparative Examples 3 and 4prepared using negative-electrode active materials for rechargeablelithium battery of Examples 1-5, Reference Examples 1 and 2, andComparative Examples 1 and 2.

DETAILED DESCRIPTION

Embodiments of the present invention will now be explained in detail.However, these are only to illustrate the invention, and the inventionis not limited thereto and it is defined by the claims.

Negative-electrode active material for rechargeable lithium batteryaccording to one embodiment of the invention comprises: a corecomprising material capable of doping and dedoping lithium; and, acarbon layer formed on the surface of the core, wherein the carbon layerhas a three dimensional porous structure. The three dimensional porousstructure comprises multiple nanopores which are regularly ordered onthe carbon layer with a pore wall placed therebetween.

The term “nanopore” herein means an internal three dimensional space atleast a part of which is surrounded by a pore wall formed by carbon ofthe carbon layer, and the diameter thereof is referred to as “nanoporediameter”. Unless there is other clear description such as “afterconducting charge/discharge” herein, the nanopore diameter means theinitial diameter of the nanopore before charge/discharge of therechargeable lithium battery comprising negative-electrode activematerial is conducted.

And, the “pore wall” means a wall formed between the nanopores to make aboundary thereof, and the “nanopore wall thickness” means the thicknessof the wall, specifically a distance or an interval between thenanopores.

And, the description “regularly ordered” means that the nanopores arearranged continuously and regularly at a specific interval, for example,with a pore wall having uniform thickness placed therebetween.

And, the three dimensional porous structure means a structure comprisingmultiple nanopores which are ordered regularly and three-dimensionallyat a specific interval of uniform pore wall thickness over at least aspecific surface area of the carbon layer.

In the negative-electrode active material, the nanopores are regularlyordered on the carbon layer with a boundary of the pore wall havinguniform thickness, and thus, when the volume of active material changesby charge/discharge of lithium, for example, the volume of “materialcapable of doping and dedpoing lithium” comprised in the coreexpands/contracts, the nanopores and nanopore wall can function as abuffer layer for the volume change.

Specifically, in the negative-electrode active material, on a carbonlayer having a thickness of several tens nm or less, multiple nanoporeshaving large diameter which could not be achieved in the prior art, forexample, an average diameter of about 100 to 300 nm, more specificallyabout 100 to 150 nm, about 150 to 200 nm, about 200 to 250 nm, or about250 to 300 nm, are ordered, and, the nanopores are regularly orderedwith a boundary of a pore wall having relatively thin thickness, forexample uniform thickness of about 40 to 150 nm, more specifically about40 to 80 nm, about 80 to 120 nm, or about 120 to 150 nm. Thus, thenanopores and the pore wall can effectively buffer volume change ofactive material occurred at charge/discharge of lithium. For thisreason, the negative-electrode active material and rechargeable lithiumbattery comprising the same can exhibit excellent capacity retentionratio and cycle life property.

And, in the negative-electrode active material, since nanopores havingrelatively large diameter are ordered regularly and three-dimensionally,the nanopores can be filled with electrolyte thereby increasing area incontact with electrolyte and facilitating doping and dedoping oflithium. Especially, since the nanopores are regularly ordered with apore wall having uniform and thin thickness placed therebetween,electrolyte of rechargeable lithium battery can uniformly diffuse at apart in contact with negative electrode surface, and the thin pore wallof uniform dimension functions for shortening the path of lithium ionand electrons at charge/discharge of rechargeable lithium battery.

Therefore, the negative-electrode active material and rechargeablelithium battery comprising the same can exhibit more improved capacityproperty and high rate property.

Meanwhile, in the negative-electrode active material, the core mayfurther comprise, in addition to the material capable of doping anddedpoing lithium, oxides thereof.

And, after conducting charge/discharge, the nanopore may have averagediameter of about 30 to 150 nm, more specifically about 30 to 60 nm,about 60 to 100 nm, or about 80 to 150 nm. Since nanopores maintainaverage diameter in the above range even after conductingcharge/discharge of rechargeable lithium battery, they can maintain moreexcellent buffering effect at volume expansion/contraction ofnegative-electrode active material.

And, after conducting charge/discharge, the thickness of the pore wallmay be about 40 to 120 nm, more specifically about 40 to 60 nm, about 60to 90 nm, or about 70 to 120 nm. Since the pore wall maintains thicknessin the above range even after conducting charge/discharge, it canmaintain excellent buffering effect at volume expansion/contraction ofnegative-electrode active material, and the negative-electrode activematerial can exhibit more excellent capacity retention ratio and cyclelife property.

Although the average diameter of nanopores and thickness of pore wallchange after conducting charge/discharge, the form of thenegative-electrode active material for rechargeable lithium batteryremains unchanged, thereby maintaining excellent cycle life property.

In the negative-electrode active material, the core comprising materialcapable of doping and dedoping lithium may have average diameter ofabout 10 to 50 μm, more specifically about 10 to 20 μm, about 20 to 40μm, or about 30 to 50 μm, but not limited thereto.

And, the material capable of doping and dedoping lithium may compriseGroup 14 or 15 element-containing material, for example, one or morekinds of the material selected from the group consisting of Si,SiO_(x)(0<x<2), Si—Y₁ alloy, Sn, SnO₂, Sn—Y₂, Sb and Ge (wherein Y₁ andY₂ is one or more kinds of atoms selected from the group consisting ofalkali metals, alkaline earth metals, Group 13 atoms, Group 14 atoms,transition metal and rare earth atoms, provided that Y₁ is not Si, andY₂ is not Sn). More specifically, Y₁ and Y₂ is one or more kinds of atomcapable of binding with Si or Sn, for example, one or more kinds of atomselected from the group consisting of Mg, Ca, Sr, Ba, Ra, Sc, Y, Ti, Zr,Hf, Rf, V, Nb, Ta, Db, Cr, Mo, W, Sg, Tc, Re, Bh, Fe, Pb, Ru, Os, Hs,Rh, Ir, Pd, Pt, Cu, Ag, Au, Zn, Cd, B, Al, Ga, Si, Sn, In, Ti, Ge, P,As, Sb, Bi, S, Se, Te and Po.

The structure of the core comprising the material capable of doping anddedoping lithium can be defined by one or more peak in the X-rayphotoelectron spectroscopy graph (XPS graph). For example, in the XPS ofthe negative-electrode active material, a characteristic peakcorresponding to silicon or SiOx (0<x<2) in the core is showed at about100 eV and/or about 104 eV. Also, any substantial peak corresponding toinorganic oxide such as silica, which is used in preparation of thenegative-electrode active material, is not showed at about 105 eV andabout 110 eV. The term of “characteristic peak” means one or two peakshaving highest intensity in the XPS graph of a certain material. Also,the term of “substantial peak” means a detectable peak in the XPS graphof a certain material.

And, in the core, the material capable of doping and dedoping lithiummay exist as multiple particles, and carbon material may be furthercomprised between the multiple particles.

Each particle of the material capable of doping and dedoping lithium maycomprise crystalline grain, and the average diameter of the crystallinegrain may be about 20 to 100 nm, more specifically about 20 to 40 nm,about 30 to 60 nm, or about 60 to 100 nm. If the average diameter of thecrystalline grain is controlled in the above range, amorphous matrix cansufficiently form after conducting charge/discharge.

After conducting charge/discharge, the structure of the material capableof doping and dedoping lithium in the core may change from crystallineto amorphous. In this case, the material capable of doping and dedopinglithium in the core may have a structure comprising crystalline grainsdispersed in an amorphous matrix. The dispersed crystalline grain mayhave average diameter of about 2 to 5 nm, specifically about 2 to 3 nmor about 3 to 5 nm, and in this range, the formation of amorphous matrixis facilitated. The amorphous matrix may function for buffering atvolume expansion/contraction due to charge/discharge, thereby moreimproving cycle life property.

In the negative-electrode active material, the thickness of the carbonlayer formed on the core surface may be about 1 to 30 nm, morespecifically about 5 to 10 nm, about 10 to 15 nm, or about 15 to 25 nm.If the thickness of the carbon layer is in the above range, athree-dimensional porous structure forms satisfactorily on the carbonlayer without excessively increasing carbon content, thereby effectivelybuffering volume change of active material at charge/discharge, andthus, sufficient high rate property and sufficient capacity can beobtained.

In case the carbon layer comprises disordered carbon (low crystallinecarbon), appropriate electrical conductivity can be obtained, and theRaman integrated intensity ratio D/G(I(1360)/I(1580)) of the carbonlayer may be about 0.1 to 2, more specifically about 0.1 to 1.5, about0.5 to 1.6, or about 0.9 to 1.8. If Raman integrated intensity ratio D/Gof the carbon layer is in the above range, desired electricalconductivity can be obtained.

And, in the negative-electrode active material for rechargeable lithiumbattery, the content of carbon may be about 5 to 40 wt %, morespecifically about 5 to 15 wt %, about 10 to 30 wt %, or about 20 to 40wt %, based on the total amount of the negative-electrode activematerial. If the content of carbon is in the above range, desiredcapacity and high rate property can be obtained.

The negative-electrode active material for rechargeable lithium batterymay have specific surface area of about 50 to 200 m²/g, specificallyabout 50 to 160 m²/g, more specifically about 100 to 160 m²/g. Sincemultiple nanopores having relatively large diameter are regularlyordered on the carbon layer with a boundary of uniform and thin porewall to form the above-described three-dimensional porous structure, thenegative-electrode active material can have larger specific surface areacompared to known nanoparticle composite. Thus, the area in contact withelectrolyte increases, and doping and dedoping of lithium is activatedto exhibit excellent capacity property and high rate property. And, asexplained above, the regularly ordered nanopores and pore wall buffervolume change by charge/discharge of lithium more efficiently to achievemore improved capacity retention ratio and cycle life property. Inaddition, since the negative-electrode active material has appropriatespecific surface area range, side reaction with electrolyte is reducedto decrease irreversible capacity.

According to another embodiment of the invention, a method for preparingthe above described negative-electrode active material for rechargeablelithium battery is provided. The method comprises the steps of:modifying material capable of doping and dedoping lithium with organicfunctional groups; mixing the material capable of doping and dedopinglithium modified with the organic functional group with inorganic oxide;heating the mixture; and, removing the inorganic oxide.

As results of experiments, it was found that according to the abovemethod, a carbon layer having a scale of several tens nm or less formssatisfactorily on the surface of core comprising material capable ofdoping and dedoping lithium, and on the carbon layer, multiple nanoporeshaving large diameter of about 100 nm or more can be regularly formedand ordered, thus the negative-electrode active material forrechargeable lithium battery as explained above can be obtained.Especially, according to the method, in the step of removing inorganicoxide, multiple nanopores corresponding to the diameter of the oxideform, and thus, a three-dimensional porous structure comprising thesenanopores that are regularly ordered with a boundary of pore wall havinguniform thickness can form on the carbon layer. Therefore, by themethod, the nanopores having relatively large diameter and pore wallhaving uniform thickness can effectively buffer volume change of activematerial at charge/discharge, and large specific surface area allowspreparation of negative-electrode active material exhibiting excellentcapacity and high rate properties.

The method will now be explained in more detail.

In the method, first, material capable of doping and dedpoing lithium ismodified by organic functional groups, thereby providing materialcapable of doping and dedoping lithium modified by organic functionalgroup.

The material capable of doping and dedoping lithium modified by organicfunctional group can be made to exist in the form of viscous gel. Afterthe material capable of doping and dedoping lithium modified with theorganic functional group in the form of gel passes all the preparationprocess of negative-electrode active material for rechargeable lithiumbattery as explained below, the material capable of doping and dedopinglithium is included in a core, the organic functional group forms acarbon layer on the core surface, and the carbon layer may comprisenanopores on its surface. In the core, material capable of doping anddedoping lithium may exist as multiple particles, and carbon materialmay be further comprised between the multiple particles.

As the organic functional group, an organic group represented byC_(n)H_(m) (wherein, n and m are integer of 1 or more), specifically, afunctional group selected from the group consisting of aliphatic organicgroup, cycloaliphatic organic group and aromatic organic group can beexemplified. For example, the aliphatic organic group may be thosehaving carbon number of from 1 to 30 such as alkyl group having carbonnumber of from 1 to 30, specifically alkyl group having carbon number offrom 1 to 15; alkenyl group having carbon number of from 2 to 30,specifically alkenyl group having carbon number of from 2 to 18; or,alkynyl group having carbon number of from 2 to 30, specifically alkynylgroup having carbon number of from 2 to 18, the cycloaliphatic organicgroup may be those having carbon number of from 3 to 30 such ascycloalkyl group having carbon number of from 3 to 30, specificallycycloalkyl group having carbon number of from 3 to 18; cycloalkenylgroup having carbon number of from 3 to 30, specifically cycloalkenylgroup having carbon number of from 3 to 18; or, cycloalkynyl grouphaving carbon number of from 3 to 30, specifically cycloalkynyl grouphaving carbon number of from 5 to 18, and, the aromatic organic groupmay be those having carbon number of from 6 to 30 such as aryl grouphaving carbon number from 6 to 30, specifically aryl group having carbonnumber of from 6 to 18. More concrete examples of the organic functionalgroup include one or more kinds of functional groups selected from thegroup consisting of methyl, ethyl, propyl, butyl, cyclopropyl,cyclobutyl, cyclopentyl, cyclohexyl, and phenyl, but not limitedthereto.

The process of modifying or protecting the material capable of dopingand dedoping lithium with organic functional group is widely known inthe art, and thus, although detailed explanation thereof is not includedherein, it can be easily understood by a person having ordinaryknowledge in the art.

Then, the material capable of doping and dedoping lithium modified withorganic functional group is mixed with inorganic oxides.

After the mixing, the inorganic oxides are stuck in the surface of thematerial capable of doping and dedoping lithium modified with organicfunctional group, thereby forming a structure comprising the stuckinorganic oxides that are connected to each other.

As the inorganic oxides, one or more kinds of inactive inorganic oxidethat does not react with the organic functional group can be used. Theinorganic oxide may be one or more kinds of the inactive inorganic oxideselected from the group consisting of silica, alumina, titania, ceriaand zirconia, but not limited thereto. Commercial inorganic oxides canbe generally used.

The inorganic oxides can be nanoparticles, for example, nanoparticlehaving average diameter of about 100 to 300 nm, specifically about 100to 150 nm, about 150 to 200 nm, about 200 to 250 nm, or about 250 to 300nm. The shape of the nanoparticle can be spherical, but not limitedthereto. If the average diameter of the inorganic oxide is in the aboverange, it can be easily mixed with the material capable of doping anddedoping lithium modified with organic functional group, thus easilystuck in the surface of the material, and even at rolling after removingthe inorganic oxides, pores can be maintained without collapse.

The inorganic oxides can be added in an amount of about 10 to 80 partsby weight, specifically, 20 to 50 parts by weight, 30 to 60 parts byweight, or 40 to 80 parts by weight, based on 100 parts by weight of thematerial capable of doping and dedoping lithium modified with organicfunctional group. If the inorganic oxides are used in the above range,the negative-electrode active material for rechargeable lithium batterycan be more easily prepared.

The material capable of doping and dedoping lithium modified withorganic functional group may be in the form of viscous gel, and in thiscase, inorganic oxide nanoparticles can be easily stuck in the surfaceof the material, thereby easily forming a structure comprising the stuckinorganic oxide nanoparticles that are connected to each other. Afterpassing the step of removing the inorganic oxide nanoparticles which arestuck in the surface of the material capable of doping and dedopinglithium modified with organic functional group as explained below,nanopores regularly ordered on the carbon layer form so as to correspondto the inorganic oxide nanoparticles to make a three-dimensional porousstructure.

Meanwhile, after the mixing step, the mixture of the material capable ofdoping and dedoping lithium modified with organic functional group andthe inorganic oxides are heated.

After heating, in the material capable of doping and dedoping lithiummodified with organic functional group, organic functional groups aredecomposed thus leaving only carbon, thereby forming a carbon layer onthe surface of the material capable of doping and dedoping lithium. Theinorganic oxide particle may be stuck in the carbonlayer. The formedcarbon layer can inhibit direct reaction of the material capable ofdoping and dedoping lithium with the inorganic oxides, enabling hightemperature heating. And, in the subsequent step of removing inorganicoxides, the carbon layer can make the material capable of doping anddedoping lithium not to be easily dissolved in an acidic or basicaqueous solution.

The heating can be conducted at about 700 to 1200° C., specificallyabout 700 to 850° C., about 900 to 1100° C., or about 1000 to 1200° C.If the heating temperature is in the above range, degree ofcrystallinity of the carbon layer is excellent thus facilitating lithiumintercalation/deintercalation, and reaction of the material capable ofdoping and dedoping lithium with carbon layer can be inhibited. Forexample, if the material capable of doping and dedoping lithium issilicon, the silicon can be prevented from reacting with a carbon layerto form a non-conductor SiC in the above range of the heatingtemperature.

The heating can be conducted under vacuum or inert atmosphere, and inthis case, side reactions can be prevented. The inert atmosphere may beargon or nitrogen atmosphere, but not limited thereto.

Subsequently, the inorganic oxides are removed. Thereby, the inorganicoxide nanoparticles which are stuck in the carbon layer formed on thesurface of a core comprising material capable of doping and dedopinglithium and connected to each other can be removed. If the inorganicoxide nanoparticles are removed, multiple nanopores regularly ordered onthe carbon layer with pore wall having uniform thickness placedtherebetween can be formed.

The process of removing inorganic oxides can be conducted by addingbasic material such as sodium hydroxide, potassium hydroxide, etc. oracidic material such as HF, etc. to the heated product. This process canbe conducted for an appropriate time to remove the inorganic oxides, forexample, 2 to 3 hours.

Thereby, negative-electrode active material for rechargeable lithiumbattery according to one embodiment of the invention can be prepared.

The negative-electrode active material for rechargeable lithium batterycan be used for negative electrode of electrochemical cell such asrechargeable lithium battery. The rechargeable lithium battery comprisesa positive electrode comprising positive-electrode active material andelectrolyte, as well as the negative electrode.

The rechargeable lithium battery may show about 94% or more of coulombicefficiency after conducting 30 cycles or more of charge and discharge.The negative electrode can be prepared by mixing negative-electrodeactive material for rechargeable lithium battery, conductor, binder andsolvent to prepare a negative-electrode active material composition, andthen, directly coating it on a copper current collector and drying it.Alternatively, it can be prepared by casting the negative-electrodeactive material composition on a separate support, and then laminating afilm delaminated from the support on an aluminum current collector.

As the conductor, carbon black, graphite, or metal powder can be used,as the binder, vinylidene fluoride/hexafluoropropylene copolymer,polyvinylidenefluoride, polyacrylonitrile, polymethylmethacrylate,polytetrafluoroethylene and a mixture thereof can be used, but notlimited thereto. As the solvent, N-methylpyrrolidone, aceton,tetrahydrofuran, decane, etc. can be used, but not limited thereto. Thenegative-electrode active material, conductor, binder and the solventcan be used in an amount commonly used in rechargeable lithium battery.

The positive electrode can also be prepared by mixing positive-electrodeactive material, binder and solvent to prepare an positive-electrodeactive material composition, and then, directly coating it on analuminum current collector, or casting on a separate support andlaminating a positive-electrode active material film delaminated fromthe support on a copper current collector. The positive-electrode activematerial composition, if necessary, may further comprise conductor.

As the positive-electrode active material, materials capable ofintercalation/deintercalation of lithium are used, and for examples,metal oxide, lithium complex metal oxide, lithium complex metal sulfide,and lithium complex metal nitride, etc. can be used, but not limitedthereto.

As the separator, those commonly used in rechargeable lithium batterycan be used, and for examples, polyethylene, polypropylene,polyvinylidene fluoride or a multilayer thereof can be used, or mixedmultilayer such as polyethylene/polypropylene two-layer separator,polyethylene/polypropylene/polyethylene three-layer separator,polypropylene/polyethylene/polypropylene three-layer separator, etc. canbe used.

As the electrolyte filled in the rechargeable lithium battery,non-aqueous electrolyte or known solid electrolyte can be used, and anelectrolyte comprising lithium salt dissolved therein can be used.

As solvent for the non-aqueous electrolyte, cyclic carbonate such asethyelencarbonate, diethylenecarbonate, propylenecarbonate,butylenecarbonate, vinylenecarbonate, etc., chain carbonate such asdimethylcarbonate, methylethylcarbonate, diethylcarbonate, etc., estersuch as methylacetate, ethylacetate, propylacetate, methylpropionate,ethylpropionate, γ-butyrolactone, etc., ether such as1,2-dimethoxyethane, 1,2-diethoxyethane, tetrahydrofuran, 1,2-dioxane,2-methyltetrahydrofuran, etc., nitrile such as acetonitrile, etc., amidesuch as dimethylformamide, etc. can be used, but not limited thereto.They can be used alone or in combinations. Especially, a mixed solventof cyclic carbonate and chain carbonate can be used.

And, as the electrolyte, gel polymer electrolyte obtained byimpregnating polymer electrolyte such as polyethyleneoxide,polyacrylonitrile, etc., with an electrolyte solution, or inorganicsolid electrolyte such as LiI, Li₃N, etc. can be used, but not limitedthereto.

The lithium salt can be selected from the group consisting of LiPF₆,LiBF₄, LiSbF₆, LiAsF₆, LiClO₄, LiCF₃SO₃, Li(CF₃SO₂)₂N, LiC₄F₉SO₃,LiSbF₆, LiAlO₄, LiAlO₂, LiAlCl₄, LiCl and LiI, but not limited thereto.

EXAMPLES

The present invention will be explained in more detail with reference tothe following examples. However, these examples are only to illustratethe invention and the scope of the invention is not limited thereto.

Example 1 Preparation of Butyl-Modified Silicon

After completely mixing 30 g of SiCl₄ (purity 99.999%, Aldrich Company)and 100 g of 1,2-dimethoxyethane, the mixed solution was decanted withsodium naphthalide solution, and refluxed at 400° C. for 1 hour. Theobtained solution was mixed with 40 Ml of Grignard reagentn-butyllithium, and agitated overnight. At this time, the Grignardreagent n-butyllithium was reacted with SiCl₄ to form butyl-modifiedsilicon. The solvent and naphthalene were removed by heating to 120° C.at vacuum using a rotary evaporator, and by-products of NaCl and LiClwere removed using excessive amount of n-hexane and water. Finally,butyl-modified silicon was obtained as light yellow viscous gel.

Preparation of Negative-Electrode Active Material for RechargeableLithium Battery

The above prepared butyl-modified silicon in gel form was mixed withspherical silica nanoparticles having average particle diameter of about200 nm. The above prepared butyl-modified silicon and the sphericalsilica nanoparticles were mixed at a weight ratio of 70:30(butyl-modified silicon:spherical silica nanoparticles).

The obtained mixture was heated at 900° C. for 3 hours under Ar stream.

The heated product was immersed in 1M HF solution for 2 hours.

Thereby, negative-electrode active material for rechargeable lithiumbattery which comprises a core comprising silicon and a carbon layerhaving three-dimensional porous structure comprising nanopores formed onthe core surface was prepared. The average diameter of the nanopores wasabout 200 nm, the thickness of pore wall was about 40 nm, and thethickness of the carbon layer was about 10 nm. The average diameter ofcrystalline grain of silicon in the core was about 40 nm. And, thespecific surface area of the negative-electrode active material forrechargeable lithium battery was about 158 m²/g.

Example 2 Preparation of Negative-Electrode Active Material forRechargeable Lithium Battery

Butyl-modified silicon prepared by the same method as Example 1 andspherical silica nanoparticles having particle diameter of about 200 nmwere mixed at a weight ratio of 70:30 (butyl-modified silicon:sphericalsilica nanoparticles).

The obtained mixture was heated at 900° C. for 3 hours under Ar stream.

The heated product was immersed in 1M HF solution for 3 hours.

Thereby, negative-electrode active material for rechargeable lithiumbattery which comprises a core comprising silicon and a carbon layerhaving three-dimensional porous structure comprising nanopores formed onthe core surface was prepared. The average diameter of the nanopores wasabout 100 nm, the thickness of pore wall was about 60 nm, and thethickness of the carbon layer was about 10 nm. The average diameter ofcrystalline grain of silicon in the core was about 40 nm. And, thespecific surface area of the negative-electrode active material forrechargeable lithium battery was about 149 m²/g.

Example 3 Preparation of Negative-Electrode Active Material forRechargeable Lithium Battery

Butyl-modified silicon prepared by the same method as Example 1 andspherical silica nanoparticles having particle diameter of about 200 nmwere mixed at a weight ratio of 70:30 (butyl-modified silicon:sphericalsilica nanoparticles).

The obtained mixture was heated at 1000° C. for 3 hours under Ar stream.

The heated product was immersed in 1M HF solution for 2 hours.

Thereby, negative-electrode active material for rechargeable lithiumbattery which comprises a core comprising silicon and a carbon layerhaving three-dimensional porous structure comprising nanopores formed onthe core surface was prepared. The average diameter of the nanopores wasabout 150 nm, the thickness of pore wall was about 100 nm, and thethickness of the carbon layer was about 10 nm. The average diameter ofcrystalline grain of silicon in the core was about 60 nm. And, thespecific surface area of the negative-electrode active material forrechargeable lithium battery was about 150 m²/g.

Example 4 Preparation of Negative-Electrode Active Material forRechargeable Lithium Battery

Butyl-modified silicon prepared by the same method as Example 1 andspherical silica nanoparticles having particle diameter of about 300 nmwere mixed at a weight ratio of 70:30 (butyl-modified silicon:sphericalsilica nanoparticles).

The obtained mixture was heated at 900° C. for 3 hours under Ar stream.

The heated product was immersed in 1M HF solution for 2 hours.

Thereby, negative-electrode active material for rechargeable lithiumbattery which comprises a core comprising silicon and a carbon layerhaving three-dimensional porous structure comprising nanopores formed onthe core surface was prepared. The average diameter of the nanopores wasabout 250 nm, the thickness of pore wall was about 120 nm, and thethickness of the carbon layer was about 10 nm. The average diameter ofcrystalline grain of silicon in the core was about 60 nm. And, thespecific surface area of the negative-electrode active material forrechargeable lithium battery was about 150 m²/g.

Example 5 Preparation of Negative-Electrode Active Material forRechargeable Lithium Battery

Butyl-modified silicon prepared by the same method as Example 1 andspherical silica nanoparticles having particle diameter of about 300 nmwere mixed at a weight ratio of 70:30 (butyl-modified silicon:sphericalsilica nanoparticles).

The obtained mixture was heated at 1000° C. for 3 hours under Ar stream.

The heated product was immersed in 1M HF solution for 2 hours.

Thereby, negative-electrode active material for rechargeable lithiumbattery which comprises a core comprising silicon and a carbon layerhaving three-dimensional porous structure comprising nanopores formed onthe core surface was prepared. The average diameter of the nanopores wasabout 300 nm, the thickness of pore wall was about 150 nm, and thethickness of the carbon layer was about 10 nm. The average diameter ofcrystalline grain of silicon in the core was about 60 nm. And, thespecific surface area of the negative-electrode active material forrechargeable lithium battery was about 150 m²/g.

Reference Example 1 Preparation of Negative-Electrode Active Materialfor Rechargeable Lithium Battery

Butyl-modified silicon prepared by the same method as Example 1 andspherical silica nanoparticles having particle diameter of about 200 nmwere mixed at a weight ratio of 70:30 (butyl-modified silicon:sphericalsilica nanoparticles).

The obtained mixture was heated at 900° C. for 5 hours under Ar stream.

The heated product was immersed in 1M HF solution for 2 hours.

Thereby, negative-electrode active material for rechargeable lithiumbattery which comprises a core comprising silicon and a carbon layerhaving three-dimensional porous structure comprising nanopores formed onthe core surface was prepared. The average diameter of the nanopores wasabout 300 nm, the thickness of pore wall was about 170 nm, and thethickness of the carbon layer was about 15 nm. The average diameter ofcrystalline grain of silicon in the core was about 100 nm. And, thespecific surface area of the negative-electrode active material forrechargeable lithium battery was about 100 m²/g.

Reference Example 2 Preparation of Negative-Electrode Active Materialfor Rechargeable Lithium Battery

Butyl-modified silicon prepared by the same method as Example 1 andspherical silica nanoparticles having particle diameter of about 200 nmwere mixed at a weight ratio of 70:30 (butyl-modified silicon:sphericalsilica nanoparticles).

The obtained mixture was heated at 1000° C. for 5 hours under Ar stream.

The heated product was immersed in 1M HF solution for 2 hours.

Thereby, negative-electrode active material for rechargeable lithiumbattery which comprises a core comprising silicon and a carbon layerhaving three-dimensional porous structure comprising nanopores formed onthe core surface was prepared. The average diameter of the nanopores wasabout 80 nm, the thickness of pore wall was about 140 nm, and thethickness of the carbon layer was about 15 nm. The average diameter ofcrystalline grain of silicon in the core was about 80 nm. And, thespecific surface area of the negative-electrode active material forrechargeable lithium battery was about 100 m²/g.

Comparative Example 1

Ball milling was conducted using silicon powder (Sigma Aldrich Co., 20micron) and natural graphite at a speed of 800 rpm for 8 hours, therebypreparing carbon-coated silicon particles, which was used asnegative-electrode active material for rechargeable lithium battery. Theweight ratio of carbon and silicon in the carbon-coated silicon particlewas 44:56.

Comparative Example 2

Si_(0.7)B_(0.3) metallic solid solution powder (BET specific surfacearea 16 m²/g, average particle diameter 0.2 micron), carbon fiber (BETspecific surface area 35 m²/g, average length: 2 micron, averagediameter 0.08 micron) and 10 g of polyvinylpyrrolidone were mixed in 11of ethanol, and further mixed by wet jet mill to obtain a slurry. Thetotal weight of Si_(0.7)B_(0.3) metallic solid solution powder andcarbon fiber (CF) used for preparation of the slurry were 100 g, and theweight ratio was 70:30. Then, spray drying (at an ambient temperature of2000° C.) was applied for the slurry to form a composite particles. Theaverage particle diameter of the composite particles was 10 μm.

Subsequently, 10 g of the composite particles were heated to about 1000°C. in a boiling reactor, and the heated particles were contacted with amixed gas of 25° C. consisting of benzene and nitrogen gas, and CVDtreated at 1000° C. for 60 minutes. As result, carbon material generatedby thermal decomposition of the mixed gas was deposited on the compositeparticle to prepare negative-electrode active material.

The structure of the negative-electrode active material was confirmed bySEM photograph as shown in FIG. 1. Referring to FIG. 1, it was confirmedthat although the negative-electrode active material of ComparativeExample 2 comprises a core consisting of the composite particles and acarbon deposited layer on the core, nanopores were not formed on thecarbon deposited layer.

XPS (X-Ray Photoelectron Spectroscopy)

The negative-electrode active material for rechargeable lithium batteryprepared in Example 1 was analyzed with X-ray photoelectronspectroscopy. The result was shown in FIG. 2. For silica, X-rayphotoelectron spectroscopy shows two predominant peaks at about 110 eVand about 105 eV. However, in FIG. 2, the about 110 eV and about 105 eVpeaks were not observed, but weak peak at about 104 eV indicatingSiO_(x)(0<x<2) and strong peak at about 100 eV indicating Si wereobserved. Thus, it was confirmed that all the silica particles on thesurface of carbon layer were removed in the negative-electrode activematerial of Example 1. The strong peak at about 100 eV indicates siliconcomprised in the core, and the weak peak at about 104 eV indicates thatsome of SiO_(x)(0<x<2) exists in the core.

SEM Photograph

Immediately after heating in Example 1, SEM photographs of the compositecomprising silica nanoparticles that are stuck in the carbon layerformed on the surface of the core comprising silicon were taken, and theresults were shown in FIGS. 3a to 3c . FIGS. 3a to 3c show that silicananoparticles are stuck in the surface of the carbon layer withconnected to each other.

For the negative-electrode active material for rechargeable lithiumbattery of which silica nanoparticles connected to each other wereremoved in Example 1, SEM photographs were taken, and the results wereshown in FIGS. 4a to 4c . FIGS. 4a to 4c show that silica nanoparticlesstuck in the surface of the carbon layer with connected to each otherwere removed to produce negative-electrode active material forrechargeable lithium battery having nanopores regularly formed andordered. The average diameter of the negative-electrode active materialfor rechargeable lithium battery was about 200 nm, and the thickness ofpore wall was about 40 nm.

After 100 cycles of charge/discharge, SEM photographs of the negativematerial for rechargeable lithium battery of Example 1 were taken. Theresults were shown in FIGS. 5a and 5 b.

FIGS. 5a and 5b show that average diameter of nanopores of thenegative-electrode active material for rechargeable lithium battery waschanged to about 150 nm, and the thickness of pore wall was changed toabout 80 nm.

TEM Photograph and SADP

The negative-electrode active material for rechargeable lithium batteryprepared in Example 1 was deposited to carbon-coated copper grid toprepare a sample, and TEM photograph of the cross-section was taken. Theresult was shown in FIG. 6a . And, the result of SADP (selected areadiffraction pattern) was shown in FIG. 6 b.

In FIG. 6a , lattice fringe (111) appears. As shown in FIG. 6a , verythin disordered carbon layer was observed on the surface of thenegative-electrode active material for rechargeable lithium battery.And, as result of CHS (carbon, hydrogen, sulfur) analysis, carboncontent was 12 wt % in the negative-electrode active material forrechargeable lithium battery.

The SADP result of FIG. 6b shows the formation of diamond cubic Siphase.

After 100 cycles of charge/discharge, TEM photograph of thenegative-electrode active material for rechargeable lithium battery wastaken, and the result was shown in FIG. 7a . And, the result of SADP wasshown in FIG. 7 b.

Raman Spectrum Analysis

In order to examine ordering of the carbon layer formed on the surfaceof the core in Example 1, SERS (surface enhanced Raman spectra) of thenegative-electrode active material for rechargeable lithium battery wereanalyzed. Raman spectrum analysis was conducted using Renishaw 2000Raman microscope system and 632.8 nm laser excitation. And, in order toavoid laser heat effect, analysis was conducted at low laser output andexposure time of 30 seconds using 50-fold optical lens. The result wasshown in FIG. 8.

Peak mode at about 158 cm⁻¹ corresponding to G-mode is due to in-planedisplacement of carbon strongly coupled to hexagonal sheet, and it meansordered layer. If disorder is introduced in carbon material, bandappearing in Raman spectrum becomes broad, and disorder-induced band orD-mode band is generated around about 1360 cm⁻¹. Raman integratedintensity ratio D/G(I(1360)/I(1580)) indicates the degree ofcarbonization, and the lower intensity ratio indicates the higher degreeof carbonization.

As shown in FIG. 8, D/G(I(1360)/I(1580)) of the carbon layer of thenegative-electrode active material for rechargeable lithium battery ofExample 1 is about 1.51, which is much larger than 0.09 for orderedgraphite, indicating that disordered carbon layer forms.

X-Ray Diffraction (XRD) Analysis

For the negative-electrode active material for rechargeable lithiumbattery prepared in Example 1, X-ray diffraction analysis was conducted.The result was shown in FIG. 9.

And, after conducting 100 cycles of charge/discharge for thenegative-electrode active material for rechargeable lithium batteryprepared in Example 1, X-ray diffraction analysis was conducted. Theresult was shown in FIG. 10.

In this analysis, Cu—Kα ray was used as a light source.

As shown in FIGS. 9 and 10, in the negative-electrode active materialfor rechargeable lithium battery of Example 1, initially crystallinesilicon phase was change to amorphous silicon phase.

BET Specific Surface Area

In order to examine surface area of the negative-electrode activematerial for rechargeable lithium battery prepared in Example 1,nitrogen isothermal adsorption experiment was conducted withMicrometrics ASAP 2020 system, and the result was shown in FIG. 11. InFIG. 11, the lower line is adsorption curve of nitrogen gas, and theupper line is desorption curve of nitrogen gas.

The surface area of the negative-electrode active material forrechargeable lithium battery of Example 1 was about 158 m²/g, ascalculated using the results shown in FIG. 11 and BET(Brunauer-Emmett-Teller) equation.

Examples 6-10

The negative-electrode active materials for rechargeable lithium batteryprepared in Examples 1-5, Super P carbon black and poly(vinylidenefluoride) binder were mixed in N-methylpyrrolidone solvent at a weightratio of 80:10:10 to prepare negative-electrode active material slurry.The prepared negative-electrode active material slurry was coated on acopper foil of 50 μm thickness, and dried at 150° C. for 20 minutes, andthen, roll-pressed to prepare a negative electrode.

Using the above prepared negative electrode, lithium counter electrode,microporous polyethylene separator and electrolyte, coin type half cell(2016 R-type) was prepared in a helium-filled glove box. The half cellsprepared using the negative-electrode active materials of Examples 1-5were respectively designated as Examples 6-10. In these half cells,electrolyte obtained by dissolving 1.05M LiPF₆ in a mixed solvent ofethylene carbonate, diethylene carbonate and ethyl-methyl carbonate at avolume ratio of 30:30:40 was used.

Reference Examples 3-4

Half cells of Reference Examples 3˜4 were prepared by the same method asExamples 6˜10, except that the negative-electrode active materials forrechargeable lithium battery prepared in Reference Examples 1-2 wereused.

Comparative Examples 3-4

Half cells of Comparative Examples 3-4 were prepared by the same methodas Examples 6-10, except that the negative-electrode active materialsfor rechargeable lithium battery prepared in Comparative Examples 1-2were used.

Charge/Discharge Properties and Coulombic Efficiency

For the half cells prepared in Examples 6-10, Reference Examples 3, 4,and Comparative Examples 3 and 4, 100 cycles of charge/discharge wereconducted at 1.5 to 0V, 0.2 C (400 mA/g) to measure charge/dischargeproperties at 1, 30, 70 and 100 cycle. The results were shown in Table 1

TABLE 1 Cycle Discharge Charge Coulombic number capacity capacityefficiency Irreversible Example (time) (mAh/g) (mAh/g) (%) capacity(%)Example 1 3212 2774 86 14 6 30 2787 2760 99 1 70 2777 2722 98 1 100 26842657 99 1 Example 1 3124 2747 87 13 7 30 2767 2613 94 6 70 2697 2554 946 100 2581 2494 96 4 Example 1 3120 2746 88 12 8 30 2714 2657 97 3 702698 2611 96 4 100 2617 2587 98 2 Example 1 3117 2776 89 11 9 30 27412698 98 2 70 2699 2614 96 4 100 2597 2550 98 2 Example 1 3110 2781 89 1110 30 2787 2740 98 2 70 2711 2670 98 2 100 2654 2650 99 1

TABLE 2 Cycle Discharge Charge Coulombic Reference number capacitycapacity efficiency Irreversible example (time) (mAh/g) (mAh/g) (%)capacity (%) Reference 1 2775 2478 89 11 example 3 30 2765 2365 85 15 702536 2342 92 8 100 2465 2312 93 7 Reference 1 2764 2445 88 12 example 430 2759 2386 86 14 70 2522 2332 92 8 100 2498 2296 91 9

TABLE 3 Cycle Discharge Charge Coulombic Irrevers- Comparative numbercapacity capacity efficiency ible ca- example (time) (mAh/g) (mAh/g) (%)pacity (%) Comparative 1 2658 2335 87 13 example 3 30 2586 2320 89 11 702555 2299 89 11 100 2512 2286 91 9 Comparative 1 2648 2317 87 13 example4 30 2596 2303 88 12 70 2516 2289 90 10 100 2493 2275 91 9

From the Tables 1 to 3, it was confirmed that the half cells of Examples6 to 10 comprising the negative-electrode active materials of Examples 1to 5 exhibit excellent charge/discharge capacity (charge/dischargeproperties) and Coulombic efficiency compared to the half cells ofReference Examples 3 and 4 and Comparative Examples 3 and 4, and suchexcellent charge/discharge properties and Coulombic efficiencymaintained almost unchanged even after 100 cycles of charge/discharge.

Cycle Life Property

For the half cells prepared in Examples 6˜10, Reference Examples 3 and4, and Comparative Examples 3 and 4, 100 cycles of charge/discharge wereconducted at 1.5 to 0V, respectively at 0.2 C (400 mA/g) and 1 C (2000mA/g), and the results were shown in FIGS. 12a-12i sequentially.

As shown in FIGS. 12a-12e , for the half cells of Examples 6˜10, after100 cycles at 0.2 C, charge capacity was 2600-2780 mAh/g and capacityretention ratio was 86-99%. And, in case charge/discharge was conductedat 0.2 C, charge capacity was retained relatively stable during 100cycles (almost 100%), indicating that the formed surface layer ismaintained without damage.

And, after 100 cycles at 1 C, charge capacity was 2190˜2434 mAh/g andcapacity retention ratio was 90%. Thus, in case charge/discharge wasconducted at 1 C, charge capacity was also retained relatively stableduring 100 cycles, indicating that the formed surface layer ismaintained without damage.

Meanwhile, FIGS. 12f-12i show that for the half cells of ReferenceExamples 3 and 4 and Comparative Examples 3 and 4, after 100 cycles at0.2 C, charge capacity was less than 2400 mAh/g, which was lower thanthose of Examples 6-10, and capacity retention ratio was also low. And,after 100 cycles at 1 C, charge capacity was less than 1800 mAh/g, whichwas also much lower than those of Examples 6-10, and capacity retentionratio was also very low.

Therefore, it is confirmed that the negative-electrode active materialsof Examples 1-5 and the half cells of Examples 6-10 comprising the sameexhibit excellent capacity retention ratio and cycle life propertiescompared to those of Reference Examples and Comparative Examples.

Rate Properties

For the half cells of Examples 6-10, 1 cycle of charge/discharge wasconducted at 1.5 to 0V, respectively at 0.2 C, 1 C, 2 C and 3 C tomeasure rate properties. As results, charge capacity was 2600-2774 mAh/gat 0.2 C, 2430-2600 mAh/g at 1 C, 2150˜2373 mAh/g at 2 C, and 1875˜2015mAh/g at 3 C, and the ratio of 3 C charge capacity to 0.2 C chargecapacity was 73%.

For the half cells of Reference Examples 3 and 4 and ComparativeExamples 3 and 4, 1 cycle of charge/discharge was conducted at 1.5 to0V, respectively 0.2 C and 1 C, to measure rate properties. As results,charge capacity was about 2275-2500 mAh/g at 0.2 C, but it rapidlydecreased to about 1740-2000 mAh/g at 1 C. Thus, it was confirmed thathigh rate properties of the half cells of Reference Examples 3 and 4 andComparative Examples 3 and 4 are not satisfactory.

It is considered that these results are caused because thenegative-electrode active materials for rechargeable lithium battery ofExamples 1-5 used in Examples 6-10 comprise a carbon layer having athree-dimensional porous structure comprising nanopores of a determinedscale on its surface, and the nanopores can be filed with electrolyte tomake the area in contact with the electrolyte large, thus activatingintercalation/deintercalation of lithium. It was also considered thatregularly well-ordered nanopores enables uniform diffusion ofelectrolyte, and a thin wall of uniform dimension shortens pathway oflithium ion and electrons at charge/discharge, thereby improving highrate properties.

Meanwhile, it is expected that the negative-electrode active materialsof Reference Examples and Comparative Examples cannot exhibit theseeffects because nanopores and pore wall of scales as Examples do notform, or they do not have a three-dimensional porous structure asExamples.

The present invention is not limited to the foregoing examples anddrawings attached hereto, and various modification or alteration can bemade by a person of ordinary skill in the art without departing from theaspect and scope of the present invention as described in the claimsappended hereto.

The invention claimed is:
 1. A method for preparing negative-electrodeactive material for rechargeable lithium battery, comprising: modifyinga material capable of doping and dedoping lithium with an organicfunctional groups; mixing the modified material with an inorganic oxide;heating the mixture; and removing the inorganic oxide to form anegative-electrode active material having a core comprising the materialcapable of doping and dedoping lithium, and a carbon layer disposed onthe surface of the core, wherein the carbon layer having a threedimensional porous structure comprising nanopores having averagediameter of 100 nm to 300 nm, regularly ordered on the carbon layer witha pore wall having thickness of 40 nm to 150 nm placed therebetween. 2.The method according to claim 1, wherein the heating step comprises:heating the mixture to form a composite comprising the core and thecarbon layer disposed on the surface of the core, wherein particles ofthe inorganic oxide are disposed in the carbon layer.
 3. The methodaccording to claim 1, wherein the organic functional group isrepresented by C_(n)H_(m) (wherein, n and m are integer of 1 or more).4. The method according to claim 1, wherein the organic functional groupis selected from the group consisting of aliphatic organic group havingcarbon number of from 1 to 30, cycloaliphatic organic group havingcarbon number of from 3 to 30, and aromatic organic group having carbonnumber of from 6 to
 30. 5. The method according to claim 1, wherein theinorganic oxide comprise one or more kinds of inactive inorganic oxide,selected from the group consisting of silica, alumina, titania, ceriaand zirconia.
 6. The method according to claim 1, wherein the inorganicoxide is added in an amount of 10 to 80 parts by weight, based on 100parts by weight of the material capable of doping and dedoping lithiummodified with the organic functional group.
 7. The method according toclaim 1, wherein the material capable of doping and dedoping lithiummodified with the organic functional group is in the form of viscousgel.
 8. The method according to claim 1, wherein the step of heating isconducted under vacuum or inert atmosphere.
 9. The method according toclaim 1, wherein the heating is conducted at a temperature ranging from700° C. to 1200° C.
 10. The method according to claim 1, wherein theremoval of the inorganic oxide is conducted using basic or acidicmaterial.